The present disclosure relates to an electrostatically actuatable MEMS device.
Dielectric charging is recognized as a major reliability issue of electrostatic RF-MEMS (capacitive) devices, e.g. capacitive switches or varicaps, and in general electrostatic MEMS devices. Charges trapped in the dielectric interposer of typical parallel-plate actuators result in shifts and deformations of the ideal actuation characteristics, e.g. C-V profile, as described in X. Rottenberg, I. De Wolf, B. Nauwelaers, W. De Raedt, H. Tilmans, Analytical model of the DC-actuation of electrostatic MEMS devices with distributed dielectric charging and non-planar electrodes, Journal of Microelectromechanical Systems, Vol. 16 (5), pp. 1243-1253, 2007. In the past years it has been shown that substrate charging also plays an important role in defining the reliability of these devices, see X. Rottenberg, S. Brebels, P. Ekkels, P. Czarnecki, P. Nolmans, R. Mertens, B. Nauwelaers, B. Puers, I. De Wolf, W. De Raedt, H. Tilmans, An electrostatic fringing-field actuator (EFFA): application towards a low-complexity thin-film RF-MEMS technology, Journal of Micromechanics and Microengineering, Vol. 17 (7), pp. S204-S210, 2007, and P. Czarnecki, X. Rottenberg, P. Soussan, P. Ekkels, P. Muller, P. Nolmans, W. De Raedt, H. A. C. Tilmans, R. Puers, L. Marchand and I. De Wolf, “Influence of the substrate on the lifetime of capacitive RF MEMS switches”, Proc. IEEE Int. Conf. Micro Electro Mechanical Syst. MEMS, January 2008, Tucson, Ariz., USA, pp. 172-175.
Indeed, the substrates used for electrostatic thin-film MEMS processes are either insulating or coated with insulating materials and therefore sensitive to charges. Even though the major part of the electric field used for the actuation is usually confined to the air gap in the structure (between bridge and dielectric interposer), the substrate is also subjected to an electric stress that drives its parasitic charging. As a conclusion, it can be stated that two insulating layers influence the lifetime of typical electrostatic (RF) MEMS devices, namely the dielectric interposer and the substrate. These dielectric and charging mechanism disparities can be seen as the source for the non-uniform effective dielectric charging postulated in X. Rottenberg, I. De Wolf, B. Nauwelaers, W. De Raedt, H. Tilmans, Analytical model of the DC-actuation of electrostatic MEMS devices with distributed dielectric charging and non-planar electrodes, Journal of Microelectromechanical Systems, Vol. 16 (5), pp. 1243-1253, 2007.
Over time, these electrical charges may increase to such a level that prevents the MEMS device from returning to its steady-state position after the actuation signal is removed. That is, the MEMS device stays in its actuated position and the MEMS device fails.
In the prior art, patent application US 2006/0276015 presents a method and apparatus for reducing dielectric charging in MEMS structures. The application provides a semiconductor device which comprises a combination of a doped semiconductor layer and an insulation layer. This combination operates to provide a path to dissipate any excess electrical charge received by the insulation layer.